The present invention relates generally to a system for using ozone to treat ballast water during transit of a sea vessel from one port to another. More specifically, the invention is related to a method and apparatus for controlling an ozone generator voltage.
Sea vessels routinely use ballast tanks to maintain balance when traveling empty, the weight of the ballast water being used to compensate for the lack of cargo weight when the cargo load is empty or partial. For example, in a typical transport operation, a vessel may travel to a first port with a load of cargo and relatively little ballast water. When it reaches the first port, it takes on ballast water at the same time as the cargo is unloaded. The vessel then travels to a second port to receive cargo, where it discharges the ballast water.
One effect of this type of operation is that various organisms may be transported by the vessel between ports in the ballast water. The water typically contains algae and zooplankton, among other organisms, that may be indigenous to the cargo-discharge port but not to the cargo-loading port. There are examples where the movement of ballast water by vessels has caused harmful nonindigenous organisms to be transported to ports. One of the more striking examples is the introduction of the freshwater zebra mussel into the Great Lakes of North America in 1986.
Zebra mussels originated in the Balkans, Poland, and areas of the former Soviet Union, and were introduced through the transport of ballast water to Lake St. Clair, a small Canadian water body connecting Lake Huron and Lake Erie, in 1986. The natural predators of zebra mussels, some diving ducks, freshwater drum, carp, and sturgeon, are not sufficiently numerous in North America to have any significant impact on limiting their increase due to reproduction. As a consequence, there has been a large increase in zebra mussel population since its introduction, with the spread having been documented into Lake Erie, Lake Michigan, and numerous rivers. The behavior of the organism includes colonization of water intake pipes, and has been documented to reduce the intake of water-treatment plants, hydroelectric and nuclear power plants, and water-supply plants by as much as two thirds. In addition, zebra mussels threaten the existence of indigenous species, such as the winged mapleleaf clam, which is rendered unable to open its shell to eat when large numbers of zebra mussels colonize the shell.
The cost of attempting to control the freshwater zebra mussel had been estimated at $5,000,000,000 by 1995. Numerous informal efforts have been organized to try to identify and limit the further spread of the organism. At a formal level, the U.S. Congress passed the National Invasive Species Act (P.L. 104-332) in 1996 to arrest the spread of nonindigenous species by water-ballast discharge. Among other provisions, the Act requires the development of national guidelines to prevent the introduction of organisms into U.S. waters through movement of ballast water. One approach has been to have a vessel undertake ballast exchange on the high seas. In addition to the time and manpower needed to perform this operation without the benefit of simultaneously loading and/or unloading cargo, the emptying of ballast tanks on the high seas may create dangerous conditions, particularly in poor weather.
Embodiments of the invention are thus directed to a ballast-water treatment system. In one embodiment, an ozone generator is operated by a supply of voltage. A voltage controller regulates the supply of voltage provided to the ozone generator, and an ozone-transport system is provided to convey ozone from the ozone generator to a ballast tank. In certain embodiments, the system also includes a pressure generation system. The pressure generation system is configured to regulate a flow pressure such that the flow pressure is substantially ambient at an exit end of the ozone generator and has a positive pressure when reaching the ballast tank. In one such embodiment, the pressure generation system includes a plurality of pump sets connected in parallel. Each of the pump sets in turn includes a plurality of pumps connected in series.
Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings.